The invention relates to a process and apparatus for generating high-purity nitrogen by low-temperature fractionation of air, including both a rectification system for nitrogen/oxygen separation, and a high-purity nitrogen column, in which the high-purity product is generated from a nitrogen fraction which has been obtained in the rectification system for nitrogen/oxygen separation.
The rectification system for nitrogen/oxygen separation may be designed as a one-column, two-column or multi-column system. It is preferable to use a conventional Linde double-column process. The principles of the low-temperature fractionation of air in general and the structure of double-column installations specifically are known from the monograph xe2x80x9cTieftemperaturtechnikxe2x80x9d [Low-temperature technology] by Hausen/Linde (2nd edition, 1985) or from an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, Page 35). In addition to the rectification system for nitrogen/oxygen separation, further apparatus for obtaining other constituents of air, in particular high-purity oxygen or inert gases, such as for example argon, maybe used in the process according to the invention.
A process for obtaining high-purity nitrogen with a reduced CO content by rectification is known from European patent EP 299364 B1. The removal of CO and, if appropriate, the removal of argon in this case takes place in the upper region of the high-pressure part of the double column for nitrogen/oxygen separation. A drawback of this process is that only a small part of the overall nitrogen product can be obtained in high-purity form; most has to be tapped off as nitrogen of ordinary purity, in particular without a reduction in the CO content (and if appropriate in the argon content).
The invention is based on the object of providing a process and an apparatus which allow a particularly high proportion of the nitrogen product to be obtained in high-purity form, in particular with a reduced CO concentration.
This object is achieved by a process for a generating high-purity nitrogen by low-temperature fractionation of air in a rectification system for nitrogen/oxygen separation, which has at least a first rectifier column (4), in which process
a. cycle nitrogen (24) in gas form is removed from the upper region of the first rectifier column (4), and
b. is compressed in a cycle compressor (30),
c. a first part (35) of the compressed cycle nitrogen is liquefied,
d. a nitrogen fraction (52) from the rectifier system for nitrogen/oxygen separation is introduced (52) into a high-purity nitrogen column (39) which has a top condenser (54),
e. high purity nitrogen (56) is removed from the upper region of the high-purity nitrogen column (39), and
f. the refrigeration demand of the top condenser (54) of the high-purity nitrogen column (39) is at least partially covered by liquefied cycle nitrogen (38).
Thus, a high-purity nitrogen column whose refrigeration demand is covered by the liquid nitrogen which is generated in a nitrogen cycle is used. A cycle of this type is used to generate large quantities of liquid product and is known per se. A significant concept of the invention is the advantageous connection of this liquefaction cycle to the high-purity nitrogen column.
To transfer the refrigeration from the liquefied cycle nitrogen to the top fraction of the high-purity nitrogen column, the following variants are possible and can also in principle be implemented in any combination:
i) direct introduction of the liquefied cycle nitrogen into the evaporation space of the top condenser of the high-purity nitrogen column
ii) introduction of the liquefied cycle nitrogen into the high-purity nitrogen column (at the bottom or a few plates above it), removal of a liquid from the high-purity nitrogen column (for example at the bottom) and introduction of this liquid (the composition of which is very similar or identical to that of the liquefied cycle nitrogen) into the evaporation space of the top condenser of the high-purity nitrogen column
iii) introduction of the liquefied cycle nitrogen into another vessel (for example the first rectification column), removal of a liquid of identical or similar composition from this vessel and introduction of this liquid (the composition of which is very similar or identical to that of the liquefied cycle nitrogen) into the evaporation space of the top condenser of the high-purity nitrogen column
which is in communication with a first rectifier column of the rectification system for nitrogen/oxygen separation not directly but rather via a nitrogen cycle. For this purpose, the high-purity nitrogen column is fed with gaseous cycle nitrogen, which is preferably introduced into the lower region of the high-purity nitrogen column, from the or one of the expansion turbines of the nitrogen cycle. Within the high-purity nitrogen column, the rising vapour is enriched with constituents of relatively low volatility, in particular CO and/or argon, by counter current rectification. The nitrogen product, which is of correspondingly high purity, is removed from the upper region of the high-purity nitrogen column. On account of the cycle which is present, some or preferably all of the high-purity nitrogen product can be removed in liquid form and introduced, for example, into a tank.
In the process according to the invention, the integration of the cycle and the high-purity nitrogen column allows virtually any desired degree of conversion to be achieved in the high-purity nitrogen column by suitably designing or operating the nitrogen cycle. This allows flexible adaptation of the process to meet specific customer requirements. For example, it is possible to generate the entire useable nitrogen product in high-purity form, without nitrogen of standard purity being produced as a by-product. This is particularly favourable when the products of the process arexe2x80x94as is frequently the casexe2x80x94being introduced into liquid tanks, since one tank for the high-purity nitrogen is now sufficient instead of the two nitrogen tanks for the different purities which are required according to the prior art. Moreover, the process according to the invention allows the quantity of high-purity nitrogen which is generated to be varied during operation.
Preferably, at least a first part-stream of liquefied cycle nitrogen is fed back into the rectification system for nitrogen/oxygen separation, in particular into the first rectifier column. Consequently, the refrigeration which is generated in the cycle can be used to obtain liquid products directly from the rectification system for nitrogen/oxygen separation. In this case, by way of example, liquid nitrogen of standard purity and/or liquid oxygen are generated.
The integration between circuit system and high-purity nitrogen column can be improved further by removing the gaseous charge for the high-purity nitrogen column at least partially from the nitrogen cycle. For this purpose, a second part of the compressed cycle nitrogen is expanded and introduced into a high-purity nitrogen column. The expansion of the second part of the compressed cycle nitrogen is preferably carried out in a work-performing manner.
In many cases, a particularly low concentration of highly volatile impurities, such as hydrogen, neon and/or helium is also desirable in the high-purity nitrogen product. For this purpose, it is advantageous if the cycle nitrogen is removed at least one theoretical or practical plate below the top of the first rectifier column and/or the high-purity nitrogen is removed at least one theoretical or practical plate below the top of the high-purity nitrogen column. Preferably, in each case one to five, preferably two to three what are known as barrier plates are situated at the top of the first rectifier column or of the high-purity nitrogen column. These two measures both reduce the levels of highly volatile components in the high-purity nitrogen; they maybe employed individually or in combination.
Furthermore, it is expedient if reflux for the high-purity nitrogen column is generated in a top condenser by evaporating a second part-stream of the liquefied cycle nitrogen in a top condenser of the high-purity nitrogen column against condensing top gas from the high-purity nitrogen column. The cycle nitrogen which is evaporated in the top condenser of the high-purity nitrogen column is preferably returned to the cycle compressor, for example by being mixed with the cycle nitrogen coming from the first rectifier column. A procedure of this nature also supplies the process refrigeration required to operate the high-purity nitrogen column from the nitrogen cycle. For this purpose, a slightly lower pressure must prevail in the evaporation space of the top condenser than in the top of the high-purity nitrogen column, so that the corresponding temperature difference can drive the heat transfer at the top condenser. The operating pressure at the top of the high-purity nitrogen column is, for example, equal to the pressure at the top of the first rectifier column.
The second part-stream of the liquefied cycle nitrogen may, for this purpose, be passed directly from the cycle to the evaporation space of the top condenser of the high-purity nitrogen column. Preferably, however, it is firstly introduced into the high-purity nitrogen column, than tapped off from the lower region of the high-purity nitrogen column and then fed for evaporation in the top condenser of the high-purity nitrogen column.
The first part-stream of the liquefied cycle nitrogen can also be introduced into the high-purity nitrogen column, for example together with the second part-stream. It is then likewise tapped off from the lower region of the high-purity nitrogen column and then returned to the rectification system for nitrogen/oxygen separation.
The liquefied cycle nitrogen (first part of the compressed cycle nitrogen) must be expanded upstream of the point where it is divided into the first and second part-streams, or at the point where it is introduced into the first rectifier column. This expansion step maybe carried out by means of a restrictor valve. In the process according to the invention, it is expedient if it is carried out in a work-performing manner. For this purpose, the corresponding part-stream of the cycle nitrogen, for example in the supercritical state, enters a turbine, where it is expanded, without a phase transition, to a subcritical pressure, so that it emerges from the turbine completely in the liquid phase or substantially completely in the liquid phase (gas content for example up to about 5%). Alternatively, it is also possible to feed the turbine with cycle nitrogen which is already in liquid form at subcritical pressure. Preferably, the first and second part-streams of the first part of the cycle nitrogen are together expanded in a work-performing manner, then are together introduced into the high-purity nitrogen column, and the division into the first and second part-streams then takes place downstream of the high-purity nitrogen column.
It is preferable to use a two-turbine circuit, in which a third part of the compressed cycle nitrogen is expanded in a work-performing manner and is at least partially returned to the cycle compressor, the entry temperature of the work-performing expansion of the third part of the compressed cycle nitrogen being higher than the entry temperature of the work-performing expansion of the second part of the compressed cycle nitrogen. The fraction which is processed further in the high-purity nitrogen column therefore flows through the cold turbine. The third part-stream, after the work-performing expansion, is preferably returned to the entry to the cycle compressor, for example together with the cycle nitrogen from the first rectifier column.
In principle, it is also possible for the nitrogen from the warm turbine or from both turbines to be introduced into the high-purity nitrogen column.
In this case, it is expedient if the exit pressure of the work-performing expansion of the third part of the compressed cycle nitrogen is lower than the exit pressure of the work-performing expansion of the second part of the compressed cycle nitrogen. This method of operation on the one hand allows particularly efficient operation of the two turbines in which gaseous cycle nitrogen is expanded; on the other hand, the higher pressure of the second part is utilized to operate the high-purity nitrogen column.
In the invention, by way of example the following pressures and temperatures prevail in the various process steps:
operating pressure of the first rectifier column (e.g. high-pressure part of a double column) at the top:
for example 5 to 12 bar, preferably 6 to 8 bar exit pressure of the circuit compressor:
for example 22 to 63 bar, preferably 28 to 37 bar entry pressure of the cold turbine (second part of the compressed cycle nitrogen):
for example 50 to 70 bar, preferably 58 to 63 bar exit pressure of the cold turbine:
for example 4 to 11 bar, preferably 6.5 to 8.5 bar entry temperature of the cold turbine:
for example 150 to 175 K, preferably 155 to 170 K entry pressure of the warm turbine (third part of the compressed cycle nitrogen):
for example 22 to 63 bar, preferably 28 to 37 bar exit pressure of the warm turbine:
for example 5 to 12 bar, preferably 6 to 8 bar entry temperature of the warm turbine:
for example 250 to 270 K
pressure of the cycle nitrogen which is to be liquefied:
for example 50 to 70 bar, preferably 35 to 68 bar operating pressure of the high-purity nitrogen column at the top:
for example 5 to 12 bar, preferably 6.5 to 8.5 bar pressure in the evaporation space of the top condenser of the high-purity nitrogen column:
for example 4.5 to 11.5 bar, preferably 6 to 8 bar
The invention also relates to an apparatus for generating high-purity nitrogen by low-temperature fractionation of air in accordance with Patent claim 10.